The present invention relates to a hybrid powertrain. The invention relates also to a vehicle provided with the hybrid powertrain, a method for controlling the hybrid powertrain, a computer program to enable a computer to conduct the method for controlling the hybrid powertrain, and a computer program product comprising program code of the computer program stored on a computer-readable medium.
Hybrid vehicles may be powered by a primary means of propulsion which may be a combustion engine, and by a secondary means of propulsion which may be an electrical machine. The electrical machine will be equipped with at least one energy store, e.g. an electrochemical store for electrical energy, and with regulating equipment to regulate the flow of electrical energy between the energy store and the electrical machine. The electrical machine may thus serve alternately as a motor and a generator, depending on the vehicle's operating state. When the vehicle is braked, the electrical machine will generate electrical energy which goes into the energy store. This is usually called regenerative braking whereby the vehicle is braked by the electrical machine and the combustion engine. The stored electrical energy will subsequently be used for operation of the vehicle.
A planetary gear usually comprises three components arranged for rotation relative to one another, via a sunwheel, a planet wheel carrier and a ring gear. Knowing the number of teeth which the sunwheel and the ring gear have makes it possible to determine the mutual rotation speeds of the three components during operation. One of the planetary gear's components may be connected to an output shaft of a combustion engine. This component of the planetary gear will therefore rotate at a speed corresponding to that of the engine output shaft. A second component of the planetary gear may be connected to an input shaft of a gearbox. This component of the planetary gear will therefore rotate at the same speed as the gearbox input shaft. A third component of the planetary gear is connected to a rotor of an electrical machine to achieve hybrid operation. This component of the planetary gear will therefore rotate at the same speed as the electrical machine's rotor if they are connected directly to one another. Alternatively, the electrical machine may be connected to the third component of the planetary gear via a transmission which has a gear ratio, in which case they may rotate at different speeds. The speed and/or the torque of electrical machines may be regulated steplessly. In operating situations where a desired speed and/or torque are to be imparted to the gearbox input shaft, a control unit will use knowledge of the combustion engine's speed to calculate the speed at which the third component needs to be driven to cause the gearbox input shaft to rotate at the desired speed. A control unit will activate the electrical machine to impart the calculated speed to the third component and hence the desired speed to the gearbox input shaft.
Depending on the configuration of the gearbox connected to the planetary gear it may be possible to avoid a torque break between gear steps, but separate and complicated devices in the gearbox are often required to eliminate or reduce the torque break so as to achieve a sensation of stepless gearchange.
Connecting the combustion engine's output shaft, the electrical machine's rotor and the gearbox input shaft to a planetary gear makes it possible to dispense with the conventional clutch mechanism. During acceleration of the vehicle an increased torque has to be delivered from the combustion engine and the electrical machine to the gearbox and thence to the vehicle's tractive wheels. As both the combustion engine and the electrical machine are connected to the planetary gear, the largest possible torque delivered by them will be limited by whichever of their greatest torques is less than that of the other, taking into account the gear ratio between them. In situations where the greatest torque of the electrical machine is less than the greatest torque of the combustion engine, taking into account the gear ratio between them, the electrical machine will not be able to generate sufficient reaction torque to the planetary gear, with the result that the combustion engine will not be able to transmit its greatest torque to the gearbox and thence to the vehicle's tractive wheels. The greatest torque transferrable to the gearbox is thus limited by the capacity of the electrical machine. This is also indicated by the so-called planet equation.
Using a conventional clutch which disconnects the gearbox input shaft from the combustion engine during gearchange processes in the gearbox involves disadvantages, e.g. warming of the discs of the clutch, resulting in clutch disc wear and greater fuel consumption. Moreover, a conventional clutch mechanism is relatively heavy and expensive. It also occupies a relatively large amount of space in the vehicle.
The space available for the propulsion device in a vehicle is often limited. If the propulsion device comprises a plurality of components, e.g. a combustion engine, an electrical machine, a gearbox and a planetary gear, the configuration needs to be compact. If further components, e.g. a regenerative brake device, are to be incorporated, the need for a compact configuration of the components of the propulsion device will be still greater. At the same time, the dimensions of these components have to enable them to absorb necessary forces and torques.
Certain types of vehicles, particularly heavy trucks and buses, need a large number of gear steps. This increases the number of components in the gearbox, which has also to be dimensioned to absorb large forces and torques which occur in such heavy vehicles, thereby increasing its size and weight.
The components of the propulsion device are also required to be of high reliability and high operational safety. A gearbox that comprises disc clutches is subject to wear, which affects its reliability and service life.
During regenerative braking, kinetic energy is converted to electrical energy which goes into an energy store, e.g. accumulators. A factor which affects the service life of the energy store is its number of cycles of supplying current to and absorbing current from the electrical machines. The more numerous the cycles, the shorter the energy store's service life.
In certain operating conditions it is desirable to switch the combustion engine off with the object of saving fuel and with a view to preventing cooling of its exhaust post-treatment system. The vehicle will then be propelled by the electrical machine. When a torque contribution is needed in the hybrid powertrain, or the energy store needs charging, the combustion engine has to be started quickly and efficiently.
EP-B1-1126987 discloses a gearbox with dual planetary gears. The sunwheel of each planetary gear is connected to an electrical machine and the ring gears of the planetary gears are connected to one another. The planet wheel carriers of each planetary gear are connected to a number of pairs of gears in such a way as to provide an infinite number of gear steps. Another specification, EP-B1-1280677, also discloses how the planetary gears may be bridged by a gear step provided on the output shaft of the combustion engine.
US-A1-20050227803 discloses a vehicle transmission with two electrical machines which are connected to respective sunwheels of two planetary gears. The planetary gears have a common planet wheel carrier connected to the input shaft of the transmission.
WO2008/046185-A1 discloses a hybrid transmission with two planetary gears whereby an electrical machine is connected to one of the planetary gears and a double clutch cooperates with the other planetary gear. The two planetary gears also cooperate with one another via a gearwheel transmission.